Method of preparing a tantalum carbide crystal



' June 7, 1966 G. R. BIRD ET AL 3,254,955

METHOD OF PREPARING A TANTALUM CARBIDE CRYSTAL Filed Aug. 28, 1962 2Sheets-Sheet 1 CARBON ATOMIC PERCENT IO 4000' 1 i I I +TOC T C TaC 5L+TQ L'- 2800-v T C B? 2 -5000 5 D To ToC+C I CC CC LLI E I 4500 g 2 2 mE 2400- T00 o 0.6 2 3.2 4 s 6.2 8 l0 l2 CARBON,WEIGHT PERCENT To-CSYSTEM FIG.I

Am zw i M INVENTORS ATTOR N EYS June 7, 1966 G. R. BIRD ET AL 3,254,955

METHOD OF PREPARING A TANTALUM CARBIDE CRYSTAL Filed Aug. 28, 1962 2Sheets-Sheet 2 FIG. 2

RF POWER 30 SOURCE INVENTORS ATTORNEYS 3,254,955 METHOD OF PREPARING ATANTALUM CARBIDE CRYSTAL George R. Bird, 6 Lowell Road, Concord, Mass;Leo

Brewer, Vista Del Orinda, Orinda, Calif.; and Willis E. Gray, Jr.,Irving St., Boston, Mass.

Filed Aug. 28, 1962, Ser. No. 219,996 6 Claims. (Cl. 23-208) The presentinvention relates to a method of growing single refractory metal carbidecrystals.

It is a primary object of the present invention to provide a method forgrowing refractory metal carbide single crystals by zone melting acarbide-carbon eutectic.

It is an additional object of the present invention to provide such amethod wherein a carbon crucible makes up the second phase of theeutectic and contains a first phase metal carbide charge therein.

It is a further object of the present invention to provide such a methodwherein the carbide-carbon eutectic is zone melted by induction heatingin an inert atmosphere which acts to retard evaporation of the carboncrucible.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

.The invention accordingly comprises the several steps and the relationand order of one or more of such steps with respect to each of theothers, and the product possessing the features, properties and therelation of elements which are exemplified in the following detaileddisclosure and the scope of the application of which will be indicatedin the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings wherein:

FIGURE 1 is a diagram of a refractory metal carbidecarbon system, whichin this case is the tantalum-carbon system;

FIG. 2 is a diagrammatic illustration of the apparatus and therelationship of the several parts thereof used in the process of thepresent invention;

FIG. 3 is a cutaway perspective view of the carbon crucible during theprocess of the present invention; and

FIG. 4 is an enlarged view of a single crystal lamp filament obtained bythe method of the present invention.

Previous efforts to grow large single crystals of refractory metalcarbides, particularly of TaC, have been largely unsuccessful due to thefact that such carbides have an extremely high melting point and tend todisassociate to gaseous carbon and either subcarbides or metal duringattempted single crystal growth. The present invention proposes aprocess wherein the single crystal growth is accomplished in theeutectic composition region of the Ta-C system whereby a much lowertemperature is required to melt the carbide. Philip McKenna in MetalProgress, volume 36, 1939, described a method whereby minute singlecrystals of TaC were obtained. The method used in that case was thedissolving of tantalum in molten aluminum with carbon present and thenheating the mass to approximately 2000 C. in a graphite crucible. TheTaC crystals thus formed were isolated by treating the mass with acid toremove th aluminum and aluminum carbide. A problem with this method isits inability to grow a single crystal of T aC having any significantsize, the crystals .obtained thereby being tiny flakes having a smallsurface area with negligible depth. The present invention uses a zoneheating technique to obtain single crystal growth from a systemcomprising polycrystalline TaC contained in a bored carbon rod cruciblewhich is heated in an inert gas atmosphere by an induction heatingmethod. While the present invention uses an induction heating method,other methods Patented June 7, 1966 known in the art, such as areimaging, which create a narrow zone of heating, could also be used. Thepractice of zone melting as a metallurgical technique is also known inthe art. However, the present invention is distinguished in that itincludes a grouping of elements containing a first and second phase orcomponent of the tantalum carbon system comprising a compound whichcontrols the composition of the system by.means of the carbon crucible,which crucible not only provides a means for holding the first componentbut also functions as the second component in the Ta-C system.

Referring now to FIGURE 1, there is shown the tantalum carbon systemaccording to Finley H. Ellinger, which was first proposed inTransactions of the ASM, volume 31, March 1943. It is apparent from thisfigure that tantalum forms two carbides; Ta C with a 3.2 weight percentof carbon and TaC with a 6.2 weight percent carbon. The system also hastwo eutectic composition regions present, a Ta-Ta C eutectic whichoccurs at 0.6 weight percent carbon and 2800 C. and a TaC-graphiteeutectic which is formed at 10 weight percent carbon and a temperatureof 3300 C. The melting point of TaC has been variously proposed betweenthe regions of 3540 C. (Geach and Jones, Plansee Proc., -90, 1955) and3880 i C. (Agte and Alterhum, Z. Tech Physik, 11, 1930). It is believedthat the correct melting point is in the range of 3880 C. or higher inview of experimental results obtained in conjunction with thedevelopment of the present invention. Because of the fact that themelting point of the refractory metal carbide, TaC, is so high, thepresent invention proposes that the process of growing single TaCcrystals be carried out within the second eutectic composition region orthe TaC-graphite eutectic. This eutectic melts at a temperature of 3300C. which, while high, is still substantially lower than the estimatedmelting point of the TaC phase.

As seen in this system diagram, the pure tantalum phase, which has amelting point of 2996 C., goes into a eutectic phase with the carbondeficient carbide Ta C with the addition of a small carbon weightpercent. The addition of further increasing amounts of carbon will causea movement of the mass over into the TaC and then the TaC-C eutecticregion. Thus by use-of a graphite crucible as the second phase of themass used in the present process, it is conceivable that a first phasecharge of solid or powdered tantalum might be used, with conversion bycarburization to TaC and then formation of the TaC-C eutectic. Similarlytantalum and carbon might be used as the first phase charge, withcarburization of the tantalum taking place. A preferred method for thepresent process, however, uses powdered TaC as a first phase charge.This reduces the requirement of carburizing a tantalum or tantalumdeficient phase as would be necesary in the other suggested firstphases.

FIG. 2 illustrates the apparatus and physical relationship of theseveral parts of the present invention. The graphite or second phase ofthe eutectic region in'which the processing is made to take place issupplied by the use of a carbon crucible 10. This crucible is in theform of a bored carbon rod. The hollow core 12 formed within the rod ispreferably filled with powdered polycrystalline TaC as a first phasecharge 14 or may alternatively be filled with powdered tantalum orpowdered tantalum and carbon in combination as described above. Zonemelting is accomplished by the use of an induction heater 16 whose fieldmay be moved relative to the longitudinal axis of the crucible. Thecrucible is provided with an advancing means 18 by which it is movedthrough a water cooled induction coil 20, in the direction indicated byarrows 28, in the preferred method of the present invention. A powerinput sufiicient to raise the mass to the TaC-C eutectic melting pointis supplied by means of an RF power source 30. The power requirement foreffective melting will vary with the physical diameter of the mass.Because of the fact that the crucible is evaporating while the processof growing the single crystal is taking place, an inert gas atmosphere22 is provided about the crucible to retard evaporation. A preferredinert gas for this atmosphere is argon. Other members of the inert gasfamily such as helium and krypton might also be used. In one form of thepresent invention, successful results have been obtained using a boredcarbon rod crucible having an outside diameter of approximately 7 inchand an inside diameter of 4; inch to provide a hollow cylinder in whichthe charge comprising the first phase is inserted. The crucible in thiscase had a height of approximately 2 inches. It will be realized thatthe largest crystal which can be formed is primarily dependent upon thecapacity of the power heating device. Additions to available spacewithin the gaseous chamber'and the apparatus used for advancing thecrucibble through the zone heating means may be made with a relativelysmall additional expense. The crucible may be openended at the basethereof as shown in FIG. 3, or it may be solidly formed at the endthereof. In the former case the advancing support means 18 shown in FIG.2 would form a base for the crucible to prevent the charge 14 fromfalling out of the core 12 of the crucible.

A cutaway view of the interior of the crucible during the growth processis shown in FIG. 3. The gaseous atmosphere is not shown in this figurebut is assumed to be present to accomplish the desired results. Asshown,

the carbon crucible decomposes as the zone melting progresses along thelongitudinal axis of the crucible in the direction shown by arrow 32.This is due in part to combination with the TaC phase and in part due tothe escape of carbon atoms into the atmosphere. The induction heatingdevice, which is not shown in this figure, is held at the top region ofthe crucible until a temperature of 3300 C. is reached wherein the first(14) and second (10) phases combine to form a eutectic composition 24comprising the TaC phase and carbon. The eutectic composition at thetemperature of 3300 C. with a 10 weight percent of carbon becomesmolten. Continued heating at this temperature will cause excess carbonto be driven off from the molten mass, causing the weight percent ofcarbon within the mass to decrease. A slight decrease of the weightpercent of carbon will shift the mass into the liquid +TaC phase regionas shown in FIGURE 1. As the mass moves into this region a solid crystalof tantalum carbide will begin to form. By careful control of thetemperature, and movement of the heating device as described below, asingle crystal of tantalum carbide is initially formed from the mass andis subsequently grown by extending its lattice structure down throughthe core charge as the core is progressively zone melted. As the heatedarea progresses, that portion which has been molten, when cooled, willform a single crystal of tantalum carbide. The heating device isprogressively moved along the crucible at a predetermined rate tostimulate growth of the single crystal. Melting is accomplished by theresistance of the mass to passage of secondary current induced in themass charge by electromagnetic induction, the coil being supplied withcurrent from the RF power supply 30. The induction method of heatingalso acts to ensure homogeneity of composition by a stirring actioncaused by the induced lines of force. The molten region containing theeutectic composition 24 is progressively moved down the length of thecrucible. A single crystal 26 of tantalum carbide is thus grown and isconstantly solidifying above the molten region, which latter region isheld in place by the crucible walls as the heating element is moveddownward. Decomposition of the crucible is not so rapid as to allow thismolten region to escape from within the confines of the crucible coreand does not affect the solidified single crystal which isself-supporting.

The ability to produce single crystals of refractory metal carbideshaving an appreciable size is of particular interest in the manufactureof lamp filaments. Single crystals having a diameter of inch and alength greater than V inch have been produced utilizing the method ofthe present invention. Of course, for lamp filament use, such crystalrods would normally have to be drawn down to a diameter of less than 10mils. A typical filament shape 34 of such a single crystal refractorymetal carbide wire is shown in FIG. 4. Filament structures of thisnature and the details of their supporting structure and operatingenvironment are more fully described in US. Patent No. 3,022,437, issuedFeb. 20, 1962 in the name of Dexter P. Cooper, Jr. and in copending US.application Serial No. 5,525, filed Jan. 29, 1960, now Patent No.3,022,437, in the name of Dexter P. Cooper, Jr.

The particular advantages of the single crystal structure in a wirefilament derives from the fact that in polycrystalline structured metalcarbide filaments, oxides such as thoria or alumina must often be addedto lengthen the individual grains whereby the grain boundaries makesmaller angles with the wire axis and the probability of failure byoffset is reduced. A single crystal grain wire obviates this problem andalso gives a much stronger material having a higher brittle failure.

The process of the present invention is thus seen to provide a practicalmethod of growing single tantalumcarbide crystals. The process hereinproposed involves the combination of three primary techniques: the useof a carbon crucible as the second phase of the composition; the use ofa zone melting technique; and finally, the use of an induction heater.While the illustration used in the above description has primarily beenthe tantalum-carbon system, it should be understood that any of therefractory metal carbidesystems are similarly compatible for the growthof single crystals as described in the process of the present invention.

Since certain changes may be made in the above process and productwithout departing from the scope of the invention herein involved, it isintended that all matter contained in the above description or shown inthe accompanying drawings shall be interpreted as illustrative and notin a limiting sense.

What is claimed is:

1. A method of producing single crystals of tantalum carbide, comprisingthe steps of:

preparing a two-component composition in a Ta-C system, a firstcomponent of which contains a substantial weight percent of tantalum,and a second component of which is carbon in the form of a crucible inwhich said first component is placed; and

zone melting said composition in the Tac-C eutectic composition regionof the Ta-C system.

2. The method of claim 1 wherein the said first component is selectedfrom the group consisting of solid tantalum and powdered tantalum.

3. The method of claim 1 wherein said first component is a mixture ofpowdered tantalum and carbon.

4. The method of claim 1 wherein the said first component is powderedtantalum carbide.

5. The method of claim 1 wherein the zone melting is by inductionheating in an argon atmosphere.

6. A method of producing single crystals of tantalum carbide comprisingthe steps of:

preparing a two-component composition in a Ta-C system, a firstcomponent of which contains a substantial weight percent of tantalum anda second component of which is carbon in the form of a crucible intowhich said first component is placed;

applying heat to a selective portion of said composition by means of aheating device of suificient power to raise the said composition to theTac-C eutectic composition melting point; and

advancing said heating device along the longitudinal axis of saidcrucible at a predetermined rate.

(References on following page) References Cited by the Examiner UNITEDSTATES PATENTS Kieffer 23208 X Pfann 23223.5 X 5 Sloan 23208 X FOREIGNPATENTS 6 OTHER REFERENCES Ellinger: The Tantalum-Carbon System,Transactions of American Society for Metals, vol. 31, pp. 89- 104(1943), page 98 in particular.

BENJAMIN HENKIN, Primary Examiner. MAURICE A. BRINDISI, Examiner.

A. STEWART, G. T. OZAKI, Assistant Examiners.

1. A METHOD FOR PRODUCING SINGLE CRYSTALS OF TANTALUM CARBIDE,COMPRISING THE STEPS OF: PREPARING A TWO-COMPONENT COMPOSITION IN A TA-CSYSTEM, A FIRST COMPONENT OF WHICH CONTAINS A SUBSTANTIAL WEIGHT PERCENTOF TANTALUM, AND A SECOND COMPONENT OF WHICH IS CARBON IN THE FORM OF ACRUCIBLE IN WHICH SAID FIRST COMPONENT IS PLACED; AND ZONE MELTING SAIDCOMPOSITION IN THE TAC-C EUTECTIC COMPOSITION REGION OF THE TA-C SYSTEM.